This invention relates to an electrochemical capacitor with electrodes of activated carbon.
If large capacities per volume are required in electric circuits, electrolytic capacitors are generally employed, specifically, aluminum or tantalum capacitors. In these capacitors, in which the dielectric properties of formed oxide layers are utilized, the capacity per unit of area depends on the forming voltage and reaches values of around 1 .mu.F.cm.sup.-2. An advantage of such capacitors is the high dielectric strength of up to about 500 V.
Larger specific capacities are known from electrochemistry, for example, the double-layer capacity of smooth electrodes with capacities per unit of area up to 50 .mu.F.cm.sup.-2. If such electrodes are used in a capacitor, however, the voltage at a pair of electodes, in the case of an aqueous electrolyte, must not substantially exceed 1 V in order to avoid electrolysis. The capacity of the electrodes can be increased still further if their effective surface is increased. Accordingly, activated carbon has also been proposed as a suitable electrode material (U.S. Pat. Nos. 3,536,963 and 3,652,902).
In the known electro-chemical capacitors which have at least two electrodes consisting of the same material, an insulating separator layer is arranged between the electrodes, and the space between the electrodes as well as the electrode material are filled with electrolyte. It has been found, however, that it is difficult to make thin, mechanically stable layers of activated carbon in powder form. In addition, electrodes of activated-carbon powder have a high diaphragm resistance and thus, the available capacity can be utilized only at very low frequencies.